how to wipe out the previous owners password blackberry storm?

ok its obviously stolen and most of you will say try to return it or sell it but i lost my blackberry then i got mugged and held up for my iphone....yea i know pretty scary so i got this blackberry storm for verizon..i still have a 2 year contract with at&t so i wanna use this blackberry storm that someone sold me but the previous owner has a password on it (its not the guy who sold me it, he found the phone) how can i wipeout the password and memory so i can use it...i kno on verizon phones theres no sim card but i saw videos of people putting the at&t and t mobile sim card in it and it wo thank you and please take this seriously im not gunna find the owner cause i dont want to drop more money on a new phone